1. Field of the Invention
The present invention relates generally to assessing failures of fuel rods, particularly in a core of a nuclear reactor.
2. Description of the Related Art
Fuel assemblies, such as in a nuclear core reactor, are essential nuclear components regarding the operation and safety of a plant. Fuel assemblies typically each provide a plurality of vertically upstanding fuel rods. Accordingly, recent experience in the nuclear industry is to increase fuel assembly output power, discharge exposure and cycle length as much as possible. As a result, this raises the question of fuel rod integrity and radioactivity release, especially during operational power maneuvers. Thus, monitoring of fuel rods to determine when certain operating conditions exceed acceptable tolerance levels may be crucial in a nuclear reactor. The failure of a single fuel rod in a reactor may result in the total failure of the reactor's ability to continue normal operation which may instigate the temporary shutdown of a facility or plant. A typical failure can be related to duty (i.e., hoop stress) applied on the fuel rods caused by power increases. This duty may typically occur suddenly, such as during a single power maneuver of the reactor, or may be the result of cycle loading through multiple power increases and decreases on a given fuel rod. Further, the fuel assemblies typically will have to be repaired (i.e., by replacing fuel rods within the failed assembly) or replaced, adding to the shutdown time and associated cost. Procedures for maintaining fuel rods within fundamental safety criteria may include providing statistical/empirical evidence showing that the fuel rods in a reactor may function within a given margin of safety at or below some predetermined power level, which are intended to keep the thermal and mechanical stresses on the fuel rod cladding for all rods in the reactor core at a safe level during the life and use of the fuel (e.g., to prevent any cracking or ruptures of fuel rod cladding and subsequent leaking of contaminates). Reactor operating limits are established to ensure that reactor operation is maintained within a fuel rod thermal-mechanical design and safety analysis basis. These operating limits may be defined, for example, by the maximum allowable fuel pellet operating power as a function of fuel pellet exposure level—usually expressed in terms of the maximum linear heat generation rate (i.e., MLHGR, the maximum heat generated by a fuel rod per unit length of the rod) or change in MLHGR versus exposure or time. Direct monitoring of the fuel rod operation and subsequent calculation of fuel rod performance parameters allows for the determination of appropriate operational strategy to maintain the useful life of the fuel rods. However, current monitoring techniques are limited to the setting on the output power levels of any rod (or more locally the fuel pellets) based on an a priori assumed set of operational conditions for the fuel rod. This fuel rod output power limit is then applied regardless of the actual operational history resulting in either excess conservatism and/or an incomplete assessment of a fuel rod's margin of safety.
In large facilities, for example, a nuclear power plant, monitoring the fuel rod parameters for all fuel rods may be a complex and time consuming process (e.g., a reactor typically may have approximately 60,000 fuel rods). For example, plant personnel may be required to record, analyze, interpret, determine trends, and maintain operating condition data on the rods to ensure proper operation. It may be known to determine parameters of fuel rods, such as hoop stress, internal pressure, temperature, and fission gas release while working off-line during power shut-down. However, the determination of these rod parameters may be generally determined only after the failure had occurred. Accordingly, without knowledge of these fuel rod parameters before or during a reactor power maneuver, the characteristics of the fuel rods cannot be accounted for in the operating strategies to reduce the probability of failure. Furthermore, if a fuel rod fails, the internal environment of the fuel rod may change substantially from a normal operating fuel rod. As such, once a fuel rod has failed, the potential for the failure to worsen in extent (degradation) may become the paramount concern in operating the core until the failed fuel can be discharged because hydriding and other mechanisms resulting from loss of cladding integrity may result in failed fuel rods having much less resistance to further damage. Accordingly, models that determine the behavior of failed fuel rods are different than those that apply to normal fuel rods.